support btree compaction, as per proposal of a few days ago.  btree index
pages no longer store parent links, instead they have a level indicator
(counting up from zero for leaf pages).  The FixBTree recovery logic is
removed, and replaced by code that detects missing parent-level insertions
during WAL replay.  Also, generate appropriate WAL entries when updating
btree metapage and when building a btree index from scratch.  I believe
btree indexes are now completely WAL-legal for the first time.
initdb forced due to index and WAL changes.